A frequency converter is a device, which is typically used for controlling a motor or another load. FIG. 1 shows an example of a frequency converter connection. Typically, the frequency converter 20 consists of two converters, namely, of a rectifier 21 and an inverter 22, between which there is a DC or AC intermediate circuit 23. The rectifier 21 and the inverter 22 may also be located physically apart, and one rectifier may supply a plurality of inverters through the common intermediate circuit 23. An example of the rectifiers 21 is a diode bridge, which is inputted 40 from a DC source 10, which is an AC network of 50 or 60 Hz, for instance, and an example of the inverters 22 is a converter bridge implemented by means of transistors (e.g. IGBT, Insulated-gate Bipolar Transistor) or other semiconductors. The inverter 22 is typically used for controlling the power transferred from the intermediate circuit 23 of the frequency converter to the motor 30. In the FIGURE, the supply connection 50 between the inverter 22 and the motor 30 is a three-phase DC connection, for instance. The inverter allows the motor 30 to be controlled in a reliable manner such that the motor executes accurately a desired speed or torque instruction, for instance.
The frequency converter 20 typically comprises protection diagnostics 50 for ground faults. This may be implemented in the actual frequency converter or by means of an external unit or units. For instance, a one-phase ground fault occurring in a cable between the frequency converter 20 and the motor 30 or in the motor 30 causes a fault current in a network grounded on the input side, which fault current may damage the frequency converter. The function of the ground fault protection is to detect a ground fault situation at the output of the frequency converter, for instance, in the motor supplied by the frequency converter or on the supply connection between them, and to perform an ground fault alert and/or necessary switching operations to protect the frequency converter and the devices connected thereto.
A ground fault situation can be detected by monitoring the sum Iu+Iv+Iw of the output phase currents of the frequency converter, and if it deviates from zero, a ground fault trigger is performed. In an ideal case, the sum of the currents of the output phases is zero in a normal operating situation, because loads do not typically comprise a separate return conductor, but all the current passing to the motor returns along feed conductors. In practice however, the sum of the output phase currents during normal operation is not generally zero, but it deviates to some extent therefrom, depending, for instance, on a high voltage change rate occurring at the frequency converter output, on ground capacitances of the motor and the conductor between the frequency converter and the motor as well as on asymmetry. Consequently, the threshold value of the sum current, whereby a ground fault is detected, must be set to differ from zero. Prior art arrangements for detecting a ground fault have been disclosed in publications U.S. Pat. No. 5,214,575, U.S. Pat. No. 7,154,277 and US 2005/0099743.
A problem with the above-described arrangement is that the deviation of the sum current from zero in a normal operating mode depends on the characteristics of the system, such as the length and type of the conductor between the frequency converter and the motor, and hence the sensibility of ground fault protection is not necessarily the best possible in all systems as a predetermined threshold value for the sum current is employed, if the characteristics of the system are not accurately known as the threshold value is determined.